1. Field of the Invention
The invention generally relates to coating materials for implants that are used as bone or dental substitutes. More specifically, the invention generally relates to coating materials that include a bone growth promoting agent.
2. Description of the Relevant Art
A goal of the placement of an endosseous implant is to achieve osseointegration of bone with the implant. “Osseointegration” is herein defined as the direct contact of the loaded implant material with living bone. Previously, materials were sought which would act as inert substances, usually eliciting a fibrous encapsulation around them. Osseointegration cannot occur, however, when a fibrous layer is present. It is believed that the biological response of bone is not inertness towards foreign materials, but rather integration of the material with the bone as if it were the part of the body.
In general, only the surface of an implant is in direct contact with host tissue. Thus, an implant surface may play a role in determining biocompatibility of the implant with the surrounding bone. Modifications of the implant surface may be employed as a means of controlling cellular responses to biomaterial surfaces and the process of induction of bone generation.
Plasma sprayed hydroxyapatite (“HA”) has been applied to titanium implants to enhance osteoconduction. The rationale for coating implants with HA grew out of the desire to combine the bioactivity of HA with the strength of titanium. Studies have demonstrated that dental and orthopedic implants coated with plasma sprayed HA promote greater direct bone attachment and higher interfacial strength compared to the uncoated titanium implants. The performance of HA coated implants is attributed to more rapid osseointegration and the development of increased interfacial strength that results from the early skeletal attachment and increased bone contact with the implant surface. In vivo research indicates that HA coated implants are biocompatible and may perform better than non-coated titanium implants.
HA coatings have a number of problems, including variation in bond strength at the coating-metal interface, variation in coating thickness among vendors, variation in structural and chemical properties, and nonuniformity in coating density. These problems have made it unclear whether or not plasma sprayed HA coatings are actually beneficial to implant success. It appears, however, that these problems reflect shortcomings inherent in the plasma spray technology rather than the rationale for the coating.
There are no accepted standards for plasma spraying HA on implants, and studies indicate differences among plasma sprayed HA coatings. Conflicting animal and clinical observations have been reported as a result of variable coating quality. It has been shown that the coating processes employed by manufacturers result in different surfaces, chemistries, and structures of plasma sprayed HA coatings. These differences may alter the cascade of cellular activity and host tissue responses.
Clinical failures of plasma sprayed HA coated implants have been observed to occur due to premature degradation or delamination at the HA-metal interface, indicating a weak bond at the coating-metal interface compared to bone-HA interface. Failures have been related to the dissolution of the coatings as well as the weak coating-substrate interface. Dissolution of the coatings is dependent not only on the crystallinity of the surface, but also on the porosity of the coating. During plasma spraying, there is porosity present between, as well as within, the laminar structure of the semi-molten particles that is not squeezed out on impact with the substrate. A high porosity level tends to lead to a higher dissolution rate of the coating.
Besides implant failure caused by dissolution of the coatings, failure of implants is also the result of delamination of the coatings at the coating-implant interface. The delamination of the coatings at the coating-implant interface may be related to the thickness of the applied layer. Currently, the HA coatings of commercial implants are fabricated by a modified plasma spray process that produces a thick coating. However, a report on the measurement of the HA coating thickness revealed inconsistent coating thickness among different vendors. Some coatings were observed to have significantly expanded when compared to the coating thickness prior to implantation, indicating the presence of fluid behind and within the coatings. Such coating expansion was suggested to mark the beginning of coating breakup leading to failure. Finally, it has been shown that thin HA coatings (2 μm) have a significantly greater coating-metal interfacial strength than commercially available thick (70 μm) plasma sprayed HA.